You can't directly invoke C functions from Java (and C++ functions are even
less accessible because of C++ name mangling).  What you can do is use JNI to
implement a native method which /calls/ your pre-existing C function.

There /should/ be a useful trail in the Java Tutorial
   http://java.sun.com/docs/books/tutorial/index.html
but it seems to have vanished.  There should also be a good book downloadable
from Sun's site:
   Java Native Interface: Programmer's Guide and Specification
   Sheng Liang
but that seems to have vanished too.  So you'll probably have to buy it unless
you have better luck finding it at java.sun.com than me.

Calling a va_args function creates extra complexity.  Java (pre 1.5) doesn't
support variable-argument lists, so you'd have to pass an array of some sort to
the native method.  In 1.5 variable-argument methods have been added to Java,
but they are just a hack implemented in the compiler, and underneath all that's
happening is that an array is created and filled in before passing that to the
function.  So even in 1.5 your native method would take an array containing the
variable parameters.   You then have the problem of calling (from C or C++)
another varargs function when all you have is an array of the parameters.  If
you are lucky, there will be another version of the same function which takes
an array of parameters (plus a size).  If you are less lucky then there will be
another version which takes an explicit va_list, in which case you can (in a
machine/compiler-dependent way) create a va_list from your array (consult your
nearest C/C++ guru).  Otherwise you'll just have to write a big switch --
something like:

   switch (no_of_params)
   {
   case 1:    variadicMethod(args[0]); break;
   case 2:    variadicMethod(args[0, args[1]]); break;
   case 3:    variadicMethod(args[0], args[1], args[2]); break;
   case 4:    variadicMethod(args[0], args[1], args[2], args[3]); break;
   ....
   }

All a bit messy, I'm afraid...

If you have the budget then you /might/ find JNIWrapper or Xfunction useful.
   http://jniwrapper.com/
   http://www.excelsior-usa.com/xfunction.html
I have never used either of them, and in particular don't know whether either
can deal with varargs functions.

   -- chris